Apply modernize-use-nullptr

Setup:
- `mkdir tidy; cd tidy`
- `cmake .. -DCMAKE_BUILD_TYPE=debug -DPROC_TARGET_NUMBER=1 -DCACHE_NAME_SUFFIX=4 -DBINDIR=. -DDATADIR=. -DBUILD_BUNDLE=ON -DWITH_LTO=OFF -DOPTION_OMP=OFF -DCMAKE_EXPORT_COMPILE_COMMANDS=ON`
- `cd ..`
- `find -name '*.cc' -exec clang-tidy-3.8 -header-filter=.* -p=tidy -fix-errors -checks=modernize-use-nullptr {} \;`

rtgui/blackwhite.cc

@@ -1196,7 +1196,7 @@ void BlackWhite::setBatchMode (bool batchMode)
 
     removeIfThere (mixerVBox, RGBLabels, false);
     delete RGBLabels;
-    RGBLabels = NULL;
+    RGBLabels = nullptr;
 
     ToolPanel::setBatchMode (batchMode);
     mixerRed->showEditedCB ();